Downhill skis are used under various conditions on prepared trails, or off trails and on snow of different types, such powder snow, packed snow, or hard or even frozen snow. It is well known that, depending on its structure and hence on the nature and arrangement of the materials of which it is composed, a ski can adapt better to changes on certain types of terrain and to certain types of snow than others. Thus, in order to ski on a hard or frozen trail where good traction is required, it is important to have a responsive ski in which vibrations are eliminated. When skiing in powder snow, the responsiveness of the ski is less important. In fact, on hard snow, the mechanism for starting and making a turn, at the moment the edge "bites," brings into play the inside edge of the outside ski on the turn. The skier, by controlling the magnitude and direction of the force on that edge, makes a turn with a larger or smaller radius. On the other hand, in powder snow, the mechanism for starting a turn is completely different, since the skier makes the turn by pivoting the ends of the skis laterally around the axis of the skier.
It is therefore understandable that it is sometimes difficult to use the same skis, with optimum results, on different kinds of snow.
It has been suggested that a ski be designed with an essentially rectangular cross section and having side walls made of materials with differing mechanical characteristics. The two skis of a given pair have corresponding side walls made of different materials so that it is possible for the user, by switching his two skis, to arrange the side walls with the same characteristics either side by side or opposite one another. On powder snow, the side walls made of the more rigid material are positioned on the outside while in hard snow, by switching the two skis, they are placed side by side.
Nevertheless, while this technique does produce a certain improvement in behavior relative to traditional skis, it does not provide a ski which is perfectly adapted to maneuvers under the best conditions, on snow with very different characteristics.
It may be said that the changes in structure are not sufficient to solve this problem of versatility in skis. There is another parameter which can influence the ease with which a turn is executed in powder snow; that is the shape of the ski. It has been explained that in powder snow the ends of the skis must pivot laterally. In order to facilitate the penetration of the skis into the snow layer, it is important to limit the "cutting force" by using a sloping surface.
For this purpose, skis have been designed whose upper surface is connected to the lower surface by sloping side walls (French Patent 1,343,014. The slope angle of the side walls relative to the lower face can vary along the ski, so as to be more acute at the tip and tail than at the waist (U.S. Pat. 3,272,522). The slope of the side walls can also be asymmetrical, with one side more acute than the other (German utility model registration No. 1,939,540).
Although it is undeniable that these skis with sloping side walls improve the ease of execution of turns in powder snow, it must be noted that this slope, at the point where the foot presses down the inner edge, does not aid in the performance of the ski on hard snow because this pressure, or "bite," lacking firmness, loses its effectiveness when the turn is made.
It would therefore be appropriate for a skier who always wants to ski under optimum conditions to have several pairs of skis available, each designed for one specific kind of snow. This is difficult for financial reasons and practical reasons, which do not always allow simultaneous transportation of several pairs of skis by a single individual. As well, rapid changes in the nature of the terrain and/or the quality of the snow do not allow an immediate change of skis.
A skier is therefore generally obliged to select a ski which represents a compromise, with predominant characteristics corresponding to the type of terrain and snow that the skier prefers.